U.S. patent number 5,159,516 [Application Number 07/819,408] was granted by the patent office on 1992-10-27 for overcurrent-detection circuit.
This patent grant is currently assigned to Fuji Electric Co., Ltd.. Invention is credited to Tatsuhiko Fujihira.
United States Patent |
5,159,516 |
Fujihira |
October 27, 1992 |
Overcurrent-detection circuit
Abstract
An improved overcurrent-detection circuit for detecting
overcurrent condition of a main current flowing through a
semiconductor power device uses a constant-current device to
provide a proportionally enhanced potential difference
representative of increases in such current. The proportionally
enhanced potential difference increases the accuracy of measuring
the current flowing between first and second main-current terminals
of the semiconductor power device, thereby providing more accurate
overcurrent detection, without requiring an increase in the
accuracy of a voltage comparator. The power semiconductor device is
coupled to a current-mirror element having a shunt-current
terminal. The overcurrent-detection circuit incorporates a
constant-current device connected between the second main-current
terminal and the shunt-current terminal. The constant-current
device maintains the shunt current at a substantially constant
level, after the shunt current rises to a predetermined level. The
overcurrent-detection circuit also incorporates determining means,
which is coupled across the constant-current device and includes a
voltage comparator. The voltage comparator provides an
overcurrent-detection signal when the potential difference across
the constant-current device exceeds a predetermined voltage. The
overcurrent-detection signal is used to control the main current of
the semiconductor power device. The present invention provides an
overcurrent-detection circuit with superior overcurrent-detection
accuracy using conventional power-IC manufacturing technology.
Inventors: |
Fujihira; Tatsuhiko (Matsumoto,
JP) |
Assignee: |
Fuji Electric Co., Ltd.
(JP)
|
Family
ID: |
12805076 |
Appl.
No.: |
07/819,408 |
Filed: |
January 10, 1992 |
Foreign Application Priority Data
|
|
|
|
|
Mar 14, 1991 [JP] |
|
|
3-48499 |
|
Current U.S.
Class: |
361/18;
323/315 |
Current CPC
Class: |
G01R
19/16552 (20130101); H01L 2924/0002 (20130101); H01L
2924/0002 (20130101); H01L 2924/00 (20130101) |
Current International
Class: |
G01R
19/165 (20060101); H02H 007/10 () |
Field of
Search: |
;361/18,91
;323/315,312,277 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: DeBoer; Todd E.
Attorney, Agent or Firm: Brumbaugh, Graves, Donohue &
Raymond
Claims
I claim:
1. An overcurrent-detection circuit comprising:
a semiconductor power device having first and second main-current
terminals and a control terminal, which semiconductor power device
is subject to damage from an overcurrent;
a current-mirror device having a first terminal connected in common
with said first main-current terminal, a control terminal connected
in common with said control terminal of said semiconductor power
device, and a shunt-current terminal, and arranged for conducting a
shunt current between said first terminal and shunt-current
terminal of said current-mirror device when current flows between
said first and second main-current terminals;
constant-current means connected between said second main-current
terminal and said shunt-current terminal for maintaining a shunt
current through said constant-current means at a substantially
constant level, after said shunt current rises to a predetermined
level; and
determining means, coupled across said constant-current means and
responsive to changes in voltage drop across said constant-current
means, for providing an overcurrent-control signal when the voltage
drop across said constant-current means exceeds a predetermined
voltage;
whereby, said overcurrent-control signal may be used to control
said semiconductor power device to avoid damage from
overcurrents.
2. A device according to claim 1 further comprising:
drive-circuit means, coupled to said control terminal of said
semiconductor power device, for determining current flow through
said device; and
control-circuit means, responsive to said overcurrent-control
signal, for coupling semiconductor-power-device shut-off signals to
said drive-circuit means.
3. A device according to claim 1, wherein said semiconductor power
element is a power MOSFET.
4. A device according to claim 1, wherein said constant-current
means is a depletion-type MOSFET having source and gate terminals
coupled to said second main-current terminal and a drain terminal
coupled to said shunt-current terminal.
5. A device according to claim 1, wherein said determining means
comprises:
constant-voltage means for generating a predetermined reference
voltage; and
comparator means for comparing said voltage drop across said
constant-current means to said reference voltage;
whereby said comparator means generates said overcurrent-control
signal when the voltage drop across said constant-current means
exceeds said reference voltage.
6. A device according to claim 5 further comprising:
drive-circuit means, coupled to said control terminal of said
semiconductor power device, for determining current flow through
said device; and
control-circuit means, responsive to said overcurrent-control
signal, for coupling semiconductor-power-device shut-off signal to
said drive-circuit means.
7. A device according to claim 5, wherein said constant-current
means is a depletion-type MOSFET having source and gate terminals
coupled to said second main-current terminal and a drain terminal
coupled to said shunt-current terminal.
8. An overcurrent-detection circuit comprising:
a semiconductor power MOSFET having drain, source and gate
terminals:
a current-mirror device having a drain terminal connected in common
with said power-MOSFET drain terminal, a gate terminal connected in
common with said power-MOSFET gate terminal, and a shunt-current
terminal, said current-mirror device arranged for conducting a
shunt current between said drain terminal and said shunt-current
terminal when current flows between said drain and source terminals
of said power MOSFET:
a depletion-type MOSFET having source and gate terminals connected
in common with said source terminal of said power MOSFET, and a
drain terminal connected in common with said shunt-current
terminal, said depletion-type MOSFET arranged as a constant-current
device for maintaining a shunt current through said depletion-type
MOSFET at a substantially constant level, after said shunt current
rises to a predetermined level;
determining means, including a constant-voltage device and a
comparator coupled in series across said depletion-type MOSFET for
providing an overcurrent signal when the voltage drop across said
constant-current device exceeds a predetermined voltage generated
by said constant-voltage device;
a drive circuit, coupled to said gate terminal of said power
MOSFET, for determining current flow through said power MOSFET;
and
a control circuit, coupled to said drive circuit, for generating an
overcurrent-control signal responsive to said overcurrent signal
and coupling said overcurrent-control signal to said drive
circuit;
whereby said overcurrent-control signal may be used to control said
power MOSFET to avoid damage from overcurrents.
9. An overcurrent-detection circuit comprising:
a semiconductor power device having first and second main-current
terminals and a control terminal, which semiconductor power device
is subject to damage from an overcurrent;
a current-mirror device having a first terminal connected in common
with said first main-current terminal, a control terminal connected
in common with said control terminal of said semiconductor power
device, and a shunt-current terminal , and arranged for conducting
a shunt current between said first terminal and shunt-current
terminal of said current-mirror device when current flows between
said first and second main-current terminals;
constant-current means, comprising a first enhancement-type MOSFET
having a source terminal coupled to said second main-current
terminal, a drain terminal coupled to said shunt-current terminal
and a gate terminal, for maintaining a shunt current through said
constant-current means at a substantially constant level, after
said shunt current rises to a predetermined level;
determining means, coupled across said constant-current means and
responsive to changes in voltage drop across said constant-current
means, for providing an overcurrent-control signal when the voltage
drop across said constant-current means exceeds a predetermined
voltage; and
a gate-bias circuit, coupled across said constant-current means,
including:
a second enhancement-type MOSFET having source and gate terminals
respectively coupled to said source and gate terminals of said
first enhancement-type MOSFET, and a drain terminal;
a zener diode coupled across said source and drain terminals of
said second enhancement-type MOSFET; and
a constant-voltage source coupled across said zener diode;
whereby, said overcurrent-control signal may be used to control
said semiconductor power device to avoid damage from overcurrents
and said gate-bias circuit introduces temperature-dependent
operating variations which tend to offset such Variations
inherently affecting operation of said constant-current means.
10. A device according to claim 9 further comprising:
drive-circuit means, coupled to said control terminal of said
semiconductor power device, for determining current flow through
said device; and
control-circuit means, responsive to said overcurrent-control
signal, for coupling semiconductor-power-device shut-off signals to
said drive-circuit means.
11. A device according to claim 9, wherein said semiconductor power
element is a power MOSFET.
12. A device according to claim 9, wherein said determining means
comprises:
constant-voltage means for generating a predetermined reference
voltage; and
comparator means for comparing said voltage drop across said
constant-current means to said reference voltage;
whereby said comparator means generates said overcurrent-control
signal when the voltage drop across said constant-current means
exceeds said reference voltage.
13. A device according to claim 9, wherein said gate-bias circuit
comprises:
a second enhancement-type MOSFET having a gate terminal connected
to said gate terminal of said constant-current means, a source
terminal connected to an input terminal of a zener diode, and a
drain terminal connected via a resistor to an output terminal of
said zener diode; and
a constant-voltage source having its lower-potential side connected
to said source terminal of said second enhancement-type MOSFET, and
its higher-potential side connected via a resistor to said output
terminal of said zener diode.
14. A device according to claim 12 further comprising:
drive-circuit means, coupled to said control terminal of said
semiconductor power device, for determining current flow through
said device; and
control-circuit means, responsive to said overcurrent-control
signal, for coupling semiconductor-power-device shut-off signals to
said drive-circuit means.
15. An overcurrent-detection circuit comprising:
a semiconductor power MOSFET having drain, source and gate
terminals:
a current-mirror device having a drain terminal connected in common
with said power-MOSFET drain terminal, a gate terminal connected in
common with said power-MOSFET gate terminal, and a shunt-current
terminal, said current-mirror device arranged for conducting a
shunt current between said drain terminal and said shunt-current
terminal when current flows between said drain and source terminals
of said power MOSFET:
a first enhancement-type MOSFET having a source terminal connected
in common with said power-MOSFET source terminal, a drain terminal
connected in common with said shunt-current terminal, and a gate
terminal, said enhancement-type MOSFET arranged as a
constant-current device for maintaining a shunt current through
said enhancement-type MOSFET at a substantially constant level,
after said shunt current rises to a predetermined level;
determining means, including a constant-voltage device and a
comparator coupled in series across said enhancement-type MOSFET,
for providing an overcurrent signal when the voltage drop across
said constant-current means exceeds a predetermined voltage
generated by said constant-voltage device;
a drive circuit, coupled to said gate terminal of said power
MOSFET, for determining current flow through said power MOSFET;
a control circuit, coupled to said drive circuit, for generating an
overcurrent-control signal responsive to said overcurrent signal
and coupling said overcurrent-control signal to said drive circuit;
and
a gate-bias circuit, coupled across said first enhancement-type
MOSFET, including:
a second enhancement-type MOSFET having source and gate terminals
respectively coupled to said source and gate terminals of said
first enhancement-type MOSFET, and a drain terminal;
a zener diode coupled across said source and drain terminals of
said second enhancement-type MOSFET; and
a constant-voltage source coupled across said zener diode;
whereby said overcurrent-control signal may be used to control said
power MOSFET to avoid damage from overcurrents and said gate-bias
circuit introduces temperature-dependent operating variations which
tend to offset such variations inherently affecting operation of
said first enhancement-type MOSFET.
Description
FIELD OF THE INVENTION
The present invention relates to an overcurrent-detection circuit
to be used in conjunction with a power semiconductor device, and
more particularly to an overcurrent-detection circuit incorporated
on a single chip with a power semiconductor device and its control
circuit.
BACKGROUND OF THE INVENTION
Power semiconductor devices, such as power bipolar transistors,
power MOSFET, and IGBT, are typically utilized to control high
voltages and large currents involved in switching such devices as
power sources, solenoids, lamps, motor-controlling inverters, and
DC-motor switches. These power semiconductor devices have a "safe
operation area" (SOA) that corresponds to the magnitude and
conduction time of output currents. If a current exceeding the SOA
flows for an extended period of time, the power semiconductor
devices overheat and thermally breakdown.
In order to prevent such overcurrent conditions, the power
semiconductor devices incorporate a protection device to monitor
the output current and temperature of the power semiconductor
devices. The protection device limits or interrupts the current
flow in the case of an overcurrent or overheating condition.
FIG. 4 shows a circuit diagram of an integrated circuit
incorporating a conventional overcurrent-protection circuit. An
n-channel power MOSFET (1), used as a power semiconductor device,
is connected with a current-mirror element (2) acting as a current
sensor. The drains of both the power MOSFET (1) and the
current-mirror element (2) are connected in parallel to a common,
first main-current terminal. The gates of both elements are
connected in parallel to a common control terminal (5).
The source of the power MOSFET (1) is connected to a second
main-current terminal (4). The source of the current-mirror element
(2) is connected to a shunt-current terminal (6).
The first main-current terminal (3) is connected to the
higher-potential side of a power source (8) via load (7). The
second main-current terminal (4) is connected to the
lower-potential side of the power source (8). The power source (8)
supplies the main current, I, to the power MOSFET (1) via the load
(7). A control signal sent from a drive circuit (9) to the control
terminal (5) controls the main current.
The overcurrent-detection element (11), shown in FIG. 4, consists
of a detection resistor (12) connected between the second
main-current terminal (4) and the shunt-current terminal (6), a
constant-voltage device (13), and a comparator (14) connected
between the lower-potential side of the constant-voltage device and
the shunt-current terminal. A shunt current, i, shunted from the
main current at a predetermined ratio by the current-mirror element
(2), flows across the detection resistor (12). The constant-voltage
device (13), connected between the second main-current terminal (4)
and the comparator (14), generates a predetermined, threshold
voltage, E.sub.s. The comparator (14), connected between the
positive side of the constant-voltage device (13) and the
shunt-current terminal (6), compares the potential difference
across the resistor (12) with the threshold voltage, E.sub.s. The
output of the comparator (14) is transmitted to the control
terminal (5) via a control circuit (10) and the drive circuit
(9).
In a circuit, such as one shown in FIG. 4, combining an
overcurrent-detection circuit with a power MOSFET, the amount of
the main current, I, can be ascertained by measuring the potential
difference, E, across the detection resistor (12). Measurement of
the main current is possible because the ratio of the shunt
current, i, flowing into the current-mirror element (2), relative
to the main current, is predetermined.
FIG. 5 is a characteristic graph showing the relationship between
the shunt current, i, and the potential drop across the detection
resistor, for the circuit shown in FIG. 4. It is understood that,
for the purposes of the circuit shown in FIG. 4, the ratio i/I is
equal to 1/10,000, and the resistance value of the detection
resistor (12) is 500 .OMEGA..
In the figure, assuming the upper limit of the main-current value
in the SOA to be 2A, the main current at the SOA can be determined
to be 2A by finding the point, P1, on the curve which corresponds
with the shunt-current value of 200 .mu.A and the inter-terminal
potential difference of 0.1 V between the shunt-current terminal
and the second main-current terminal. Consequently, if the
threshold voltage E.sub.s of the constant-voltage device (13) has
been set to 0.1 V, one can identify the main current as having
reached an overcurrent state when the comparator determines that
the inter-terminal potential difference, E, exceeds the threshold
voltage of 0.1 V.
As a result of incorporating the overcurrent-detection element,
overheating and breakdown failure of a power MOSFET can be
prevented by appropriately adjusting the main-current flow to the
output signal of the comparator. An overcurrent-detection signal,
V.sub.o, is transmitted to the control circuit (10) whenever the
comparator satisfies the condition of E-E.sub.2 >0. Based on the
overcurrent-detection signal, V.sub.o, the control circuit (10)
controls via the drive circuit (9) the voltage at the control
terminal (5), thereby performing a protective operation of either
limiting or interrupting the main current, I.
If the entire circuit shown in FIG. 4 could be integrated on a
single chip with the use of conventional technology, there would be
a great economic advantage. However, even if such integration is
possible, the inherent variance or production tolerance as
affecting the accuracy of the comparator, in responding to the
offset voltage applied to ascertain whether an overcurrent
condition exists, will greatly affect the overcurrent-detection
performance of the overcurrent-detection circuit.
The variance or tolerance of a comparator made in a conventional
manufacturing process suitable for production of power IC, in
responding to the offset voltage, typically reaches .+-.10 mV. When
this figure is converted to a variance in terms of detection
accuracy of the detection or shunt current, i, in FIGS. 4 and 5,
the variance is .+-.20.mu.A. In other words, the variance or
tolerance results in a determination error of .+-.10%.
If an attempt is made to use a comparator with less variance in
responding to the offset voltage, such comparator will not be
compatible with the tolerance inherent in the conventional power-IC
production process. Manufacturing such a comparator to closer
tolerances, for example, in a separate production process involving
separate chips for the power IC and comparator, will be an economic
burden.
In attempting to find a way to reduce the effect of the variance of
a comparator in responding to the offset voltage so as to permit
satisfactory detection accuracy with a single chip, one method
which might be considered might involve raising the resistance
value of the detection resistor (12) and increasing the
inter-terminal voltage drop due to the shunt current, i. However,
increasing the resistance will affect the shunt ratio of the shunt
current, i, relative to the main current, I, thereby preventing
improvement in accuracy of overcurrent determination.
SUMMARY OF THE INVENTION
In order to solve the above problem, the present invention provides
an overcurrent-detection circuit that can minimize the effect of
comparator variance or tolerance on the accuracy of overcurrent
determination, even if a comparator utilized in the circuit has a
variance or tolerance determined by manufacturing tolerances such
as satisfactory in the production of power ICs. Thus, the present
invention is intended to enable production of an
overcurrent-detecting circuit on the same chip with a power IC,
with manufacturing tolerances that match the tolerances of the
conventional, power-IC manufacturing processes.
More particularly, the overcurrent-detection circuit is arranged to
detect an overcurrent condition in the main current of a power
semiconductor device based on the potential difference across a
constant-current means, which is arranged so as to provide a higher
rate of change of voltage drop with increased current flow in the
power device to be monitored. With a higher rate of voltage change,
the effect of comparator error is reduced. If, for example, a given
current increase produced a voltage change twice as large as
previously, the measurement error of a given comparator would be
reduced in half, providing increased control accuracy.
The power semiconductor device is disposed with first and second
main-current terminals to permit the main current to flow and a
control terminal for controlling the main current. The power
semiconductor device is coupled to a current-mirror device, which
device has first terminal connected in common with said first
main-current terminal, a control terminal connected in common with
said control terminal of said semiconductor power device, and a
shunt-current terminal to conduct a shunt current that is small
relative to said main current.
The overcurrent-detection circuit is disposed with constant-current
means connected between said second main-current terminal and said
shunt-current terminal. The constant-current means maintains a
shunt current through said constant-current means at a
substantially constant level, after said shunt current rises to a
predetermined level.
The circuit is also disposed with determining means coupled across
said constant-current means. The determining means is responsive to
changes in voltage drop across said constant-current means, thereby
providing an overcurrent-detection signal when the voltage drop
across said constant-current means exceeds a predetermined
voltage.
The constant-current means may be a depletion-type MOSFET having a
gate terminal and a source terminal connected in common with the
second main-current terminal, and also having a drain terminal
connected in common with the shunt-current terminal. Alternatively,
the constant-current means may be an enhancement-type MOSFET
connected to a gate-bias circuit. In this arrangement, the source
terminal of said enhancement-type MOSFET is connected in common
with the second main-current terminal, the drain terminal is
connected in common with the shunt-current terminal, and the gate
terminal is connected to the gate-bias circuit.
The determining means typically consists of a constant-voltage
device and a comparator. The constant-voltage device is connected
between the second main-current terminal and the comparator, and it
generates a predetermined threshold voltage. The comparator
compares the voltage drop across said constant-current means to the
predetermined threshold voltage, whereby the comparator determines
whether the main current has reached an overcurrent state. When the
voltage drop across the constant-current means exceeds the
predetermined threshold voltage, the comparator generates a
detection signal.
In the present invention, after the shunt current rises to a
predetermined level, the constant-current device suppresses
proportional increase of the shunt current as the main current
increases. As a result, with the current held substantially
constant, the voltage across the constant-current device is caused
to increase at a substantially increased rate, as compared to the
voltage increase across the detection-resistor (12) in the
prior-art circuit of FIG. 4. Consequently, the effects of the
variance or tolerance of the comparator on the accuracy of
overcurrent determination is reduced in significance as the
potential difference across the constant-current means increases
beyond the predetermined threshold voltage. In addition, the degree
of variance or tolerance of the comparator in responding to the
offset voltage can be allowed to be determined by the conventional
power-IC manufacturing process, without compromising the accuracy
in overcurrent determination.
If the constant-current means utilizes a depletion-type MOSFET
having a gate terminal and a source terminal connected in common
with the second main-current terminal, and a drain terminal
connected in common with the shunt-current terminal, it is easy to
maintain the shunt current constant in a region beyond the MOSFET's
pinch-off voltage. Consequently, the effects of variance or
tolerance of the comparator in responding to offset voltage can be
reduced. Furthermore, one can easily form via a conventional
power-IC manufacturing process an overcurrent-detection circuit
integrated on a single chip with a power semiconductor device.
If the constant-current means utilizes an enhancement-type MOSFET
connected to a gate-bias circuit, the gate-bias circuit can
maintain the gate voltage of the enhancement-type MOSFET constant.
Furthermore, the temperature dependence of overcurrent-detection
accuracy, resulting from temperature changes in circuit elements,
can be corrected.
In both embodiments of the present invention, significance of the
variance or tolerance as affecting the accuracy of the comparator
in responding to the offset voltage is minimized by the
constant-current means. Furthermore, a comparator having the
inherent tolerances of conventional power-IC manufacturing process
can be used in the overcurrent-detection circuit, without
compromising the overcurrent-detection performance.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a circuit diagram of an overcurrent-detection circuit
according to one embodiment of the present invention.
FIG. 2 is a characteristic current-voltage curve illustrating the
operation of the overcurrent-detection circuit according to the
embodiment shown in FIG. 1.
FIG. 3 is a circuit diagram of an overcurrent-detection circuit
according to another embodiment of the present invention.
FIG. 4 is a circuit diagram of a conventional overcurrent-detection
circuit.
FIG. 5 is a characteristic current-voltage curve illustrating the
operation of the conventional overcurrent-detection circuit shown
in FIG. 4.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a circuit diagram of an overcurrent-detection circuit
according to one embodiment of the present invention. In FIG. 1, an
overcurrent-detection circuit (21) consists of a constant-current
device (22) and a determining means (23).
The constant-current device in FIG. 1 is a depletion-type MOSFET.
The MOSFET has a drain terminal connected in common with a
shunt-current terminal (6) of a current-mirror element (2), and a
source terminal and a gate terminal connected in common with a
second main-current terminal (4) of the power MOSFET (1).
The determining means (23) consists of a comparator (14) and a
constant-voltage device (13). The shunt-current terminal (6) is
connected to the positive-input terminal of the comparator, and the
constant-voltage device (13) is connected between the second
main-current terminal (4) and the negative-input terminal of the
comparator. The comparator (14) compares the potential difference
across the constant-current device, E, with a predetermined
threshold voltage, E.sub.s, of the constant-voltage device. The
comparator (14) generates an overcurrent-detection signal when E
exceeds E.sub.s. The signal is sent to a control circuit (10)
controlling the voltage at a control terminal (5), thereby
controlling the overcurrent.
FIG. 2 shows a characteristic current-voltage curve illustrating
the operation of the constant-current device (22) in the
overcurrent-detection circuit (21) shown in FIG. 1. The
constant-current device (22) is a depletion-type MOSFET which
maintains the shunt current, i, constant in the curve region beyond
the pinch-off voltage, P.sub.o.
In FIG. 2, point P2 is defined such that it corresponds to the
potential difference, E, of 0.1 V across the constant-current
device when the shunt current, i, is 200 .mu.A. An equivalent
resistance between the shunt-current terminal and the second
main-current terminal, then, can be regarded as being equivalent to
the resistance of the detection resistor (12), 500 .OMEGA.,
incorporated in the conventional overcurrent-detection circuit of
FIG. 4. As previously mentioned, it is understood that, for the
purposes of FIG. 4, the constant-voltage source (13) generates a
threshold voltage of 0.1 V. Consequently, the main current of the
circuit shown in FIG. 2 can be controlled by a comparator (14) that
compares the potential difference, E, across the constant-current
device (22) with the threshold voltage, E.sub.s. The comparator
transmits an overcurrent-detection signal, V.sub.o, to the control
circuit (10) when E exceeds E.sub.s.
If the main current flowing into the power MOSFET increases, the
shunt current, i, will have tendency to increase proportionally.
However, because the constant-current device (22) maintains the
shunt current constant, the equivalent resistance across the
constant-current device increases with the increase in the main
current. Consequently, while the potential difference, E, across
the constant-current device (22) increases as a result of the
generated overcurrent, the variance or the tolerance of the
comparator (14) in responding to the offset voltage does not
change.
Furthermore, the rate of increase of the potential difference, E,
across the constant-current device (22) in FIG. 2 is generally
greater than the rate at which the shunt current, i, attempts to
increase. With a given current increase now represented by a
proportionally larger increase in potential difference, E, the
inaccuracy of comparator (14) has proportionally less effect.
Consequently, the accuracy of determining means (23) is increased
despite the accuracy of the comparator (14) remaining
unchanged.
Overcurrent-detection tests were performed on 200 power IC's
incorporating power MOSFETs with the overcurrent-detection circuit
of FIG. 2, a control circuit, and a drive circuit. The ratio of
standard deviation to the mean value of the overcurrent
measurements was compared with the ratio obtained from the
conventional circuit shown in FIG. 4. The resulting
overcurrent-detection tolerance value for the embodiment of FIG. 2
was approximately 3%, while the tolerance value for the
conventional circuit was approximately 7%.
Furthermore, the tests proved that integration of the
overcurrent-detection circuit shown in FIG. 2 was possible via
conventional power-IC manufacturing process, thereby resulting in
economic benefits.
FIG. 3 is a circuit diagram of another embodiment according to the
present invention. It differs from the embodiment of FIG. 2 in that
the constant-current device (32) is an enhancement-type MOSFET. The
drain terminal of this first enhancement-type MOSFET is connected
to a shunt-current terminal (6), and the source terminal is
connected to a second main-current terminal (4) of a power MOSFET
(1). The gate terminal of first enhancement-type MOSFET is
connected to the gate terminal of second enhancement-type MOSFET
(42) incorporated in a gate-bias circuit (41).
The gate-bias circuit (41) consists of second enhancement-type
MOSFET (42), a constant-voltage source (43), a Zener diode (45),
and resistors (44) and (46). The gate-bias circuit (41) maintains
the shunt current constant by detecting the change in the shunt
current, i, flowing between the gate and source of first
enhancement-type MOSFET (32), which change is reflected as the
change in the gate-bias voltage of second enhancement-type MOSFET.
Furthermore, the temperature dependence of the shunt current
associated with the temperature changes in the circuit elements can
be corrected by incorporating the gate-bias circuit.
Both embodiments of the present invention provide
overcurrent-detection performance with a smaller margin of error
(i.e., improved accuracy) as compared to the conventional
overcurrent-detection circuit. Consequently, no determining means
involving a more accurate, higher cost comparator is required, and
an overcurrent-detection circuit with excellent
overcurrent-detection accuracy can be provided more economically
and advantageously.
The embodiment of the present invention incorporating an
enhancement-type MOSFET has an additional advantage. This
embodiment corrects the temperature dependence of the overcurrent
resulting from temperature changes in the circuit elements, by
providing a second enhancement-type MOSFET which is subject to
offsetting temperature effects.
The embodiments described above were explained in terms of
application to a low-side-switching type circuit, in which a load
(7) is connected to the higher-potential side of the power source
(8), and the power semiconductor device is connected to the
lower-potential side of the power source. However, the present
invention should not be limited to above applications. The present
invention can also be applied to a high-side-switching type circuit
or a bridge-type circuit.
Although the embodiments described above incorporated an N-channel
power MOSFET, an overcurrent-detection circuit according to the
present invention can also be applied to a P-channel power MOSFET.
In addition, the present invention can be also applied to IGBTs and
power bipolar transistors.
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